Air suspensions utilize air springs to provide desired output characteristics, such as ride comfort and vehicle performance for example. One known active air suspension uses an air spring assembly that includes a primary airbag mounted around a piston airbag such that the piston airbag provides a rolling surface for the primary airbag. A change in piston airbag volume changes an effective piston area of the primary airbag. A relatively small change in the effective piston area provides a change in a spring rate of the air spring assembly. The pressures in the piston airbag and the primary airbag are selectively controlled to provide infinite variation in spring rates without requiring any auxiliary tanks and associated actuators. The smaller volume of the piston airbag relative to the larger volume of the primary airbag permits rapid pressure and volume changes to enable active suspension control.
Traditionally, active air suspension systems have made gross discrete changes in suspension spring stiffness through switchable air spring volumes. The active air suspension with a dual air spring configuration described above is a force and a spring stiffness adjusting device. One problem with such a system is that the natural frequency of the spring assembly changes during the active control. An improved control configuration is needed to address these changing natural frequencies.